Two-dimensional Group VI transition metal chalcogenides (TMDCs) such as molybdenum disulfide (MoS₂) have attracted tremendous attention in opto-electronics due the extraordinary properties linked to their 2D character. As they combine a high oscillator strength band-gap transition with an exceptionally huge absorption coefficient, it is no surprise that MoS₂ is widely studied as novel opto-electronic material in, e.g., LEDs and lasers. Recent progress shows that MoS₂ nanosheets can be produced with excellent size and thickness distribution through colloidal hot injection methods [1]. Such methods produce hybrid nanocrystals where both the inorganic core and the organic ligand shell can contribute to the nanocrystal properties. Notably, the ligand shell surrounding the nanocrystal has a pivotal role in the stabilization of nanocrystal dispersions. To date, the surface chemistry of MoS₂, and in general of two-dimensional TMDCs, remains however unexplored territory. In this contribution, we shed light on the surface chemistry of MoS₂ nanosheets. Additionally, we propose different approaches to stabilize such a colloidal dispersion by judicious choice of solvent and capping ligand. In first instance, we found that as-synthesized MoS₂ exhibits poor colloidal stability in commonly used solvents such as toluene and chloroform. Quantitative NMR analysis ascribes (one of) the origin(s) of colloidal instability to insufficient ligand passivation. Steric stabilization by ligand repulsion, the main stabilization mechanism for semiconductor nanocrystals, is not sufficient to stabilize colloidal 2D MoS₂ in solution. We could overcome this issue by the choice of a suitable solvent (1,2-dichlorobenzene), which ensured colloidal stability by balancing the inter-sheet attraction forces and solvent stabilizing forces. A pivotal element here has shown to be the choice of a solvent with a matching surface tension – a case rationalized within the framework of solubility thermodynamics. Indeed, similar results have been obtained in the field of liquid phase exfoliation where the exfoliation yield is maximized if the surface energy of solvent and material of interest match. Alternatively we show that post-functionalization by dynamic Z-type ligands, which could bind to the sulfur-rich top and bottom facets, forms a simple approach to stabilize colloidal 2D MoS₂  in more-conventional solvents such as toluene and chloroform. These observations do not only shed light on their stabilization, but also highlight the stringent possibility to post-exfoliate colloidaly synthesized TMDCs in apolar solvents.

[1] Son, D. et al. (2016). Colloidal Synthesis of Uniform‐Sized Molybdenum Disulfide Nanosheets for Wafer‐Scale Flexible Nonvolatile Memory. Advanced Materials, 28(42), 9326-9332.